The present invention relates to an electrochemical sensor element for determining the concentration of a gas component in a gas mixture, in particular for determining the oxygen concentration in exhaust gases of internal combustion engines.
An electrochemical sensor for determining oxygen concentration is described in German Published Patent Application No. 196 47 144, for example. The sensor elements described are used in gas detectors which are used to regulate the air/fuel ratio of combustion mixtures in automotive engines and as broadband lambda probes. A concentration cell is combined with an electrochemical pump cell in these sensor elements.
The concentration cell has a measurement electrode situated in a measuring gas area and a reference electrode situated in a reference gas area. The exhaust gas passes through a gas access orifice and a diffusion barrier to enter the measuring gas area. The reference gas area communicates with a reference atmosphere through an opening situated on the side of the sensor element facing away from the measuring gas area. The measuring gas area and the reference gas area are situated in the same layer plane of the sensor element, which is structured as a layered system and are separated by a gas-tight partition. A Nernst voltage develops between the measuring electrode and the reference electrode and can be used to determine the ratio of the oxygen partial pressure in the measuring gas area to the oxygen partial pressure in the reference gas area.
The pump cell has a first pump electrode situated in the measuring gas area and a second pump electrode situated on a surface of the sensor element facing the exhaust gas, and it pumps oxygen ions out of the measuring gas area into the exhaust gas, or conversely, out of the exhaust gas and into the measuring gas area. A pump voltage applied to the pump cell is regulated by an external circuit element, so that a predetermined oxygen partial pressure which corresponds to a certain Nernst voltage prevails in the measuring gas area. The pump voltage is selected so that the pump current flowing in the pump cell is limited by the diffusion rate of the oxygen molecules through the diffusion barrier, and the stream of oxygen molecules flowing through the diffusion barrier is proportional to the oxygen concentration in the exhaust gas, so the oxygen partial pressure of the exhaust gas can be determined from the pump current.
A disadvantage of the above-described sensor element is that the design of two gas spaces that are separated from one another in a gas-tight manner in one plane of the sensor element, namely the measuring gas area and the reference gas area, is complicated and difficult from the standpoint of the manufacturing technology.
The electrochemical sensor element according to the present invention has the advantage that the structure of the sensor element is greatly simplified by providing an additional diffusion resistor between the measuring gas area and the reference gas area. This achieves the result that it is not necessary to form a recess for the reference gas area separated from the measuring gas area in a gas-tight manner.
Since the gas component, generally oxygen, is pumped into the reference gas area by an external circuit element via the reference electrode, this achieves the result that a uniform partial pressure of the gas component prevails in the reference gas area, so that the partial pressure of the gas component in the measuring gas area may be determined with a good accuracy via the voltage difference (Nernst voltage) which develops between the measuring gas electrode and the reference electrode.
It is also advantageous that the reference gas area communicates with a gas space situated outside the sensor element only via the additional diffusion resistor. This prevents impurities from the reference gas atmosphere, for example, from entering the reference gas area, which could result in damage to the reference electrode and thus impair the sensor function.
The sensor design is greatly simplified further by the fact that the measuring gas area and/or the reference gas area is filled in at least some areas by a porous layer forming the diffusion resistor and/or the additional diffusion resistor.
Due to the fact that the reference gas area is situated in a channel-shaped area remote from the gas access orifice, this achieves the result that the concentration of the gas component in the reference gas area is influenced only slightly by fluctuations in the concentration of the gas component of the gas mixture.
Due to the fact that a measuring gas electrode and the reference electrode are situated in the same layer plane, this yields the manufacturing advantage that the measuring gas electrode and the reference electrode may be applied in a printing step.